1. Field of the Invention
The present invention relates generally to the art of wood preservation, and more particularly to the art of increasing the resistance of pressure permeable wood species to deterioration caused by weathering, leaching or attack by fungi, insects, marine borers, etc., and in certain embodiments to the art of coloring wood during the preservation process.
2. Description of the Prior Art
It has been known for many years that woods of various types may be preserved by chemical treatment. For example, lumber has been treated with creosote to prevent decay and to prevent termite attack. More recently, pentachlorophenol has been used as a preservative, as have certain proprietary mixtures of compounds of copper, chromium and arsenic referred to in the wood preservation industry as CCA. All of the aforementioned treatment agents suffer from one or more drawbacks, principally because they are highly toxic and/or hazardous to humans and/or the environment. Such hazards can cause problems: (1) during the initial treatment process (for example, soil and ground water contamination, toxic exposure to workers); (2) during transportation and installation (for example, toxic exposure during handling, contamination of transportation and installation equipment); (3) during use (for example, by soil and ground water contamination through leaching, toxic exposure to consumers); and, (4) during disposal (for example, toxic exposure during burning, soil and ground water contamination after decay). Environmental regulations and lawsuits relating to personal injuries suffered by persons exposed to treated wood products are making it impractical to continue to use such preservation agents. Moreover, prior art preservation agents have not been entirely suitable for their intended purposes because of other drawbacks. These include undesirable coloration of the treated wood, the need to use high temperatures and/or pressures in the treatment process, the use of toxic or environmentally dangerous chemicals, lack of penetration, case hardening, etc. Most importantly, however, the treating agents have not satisfactorily protected the wood against leaching of the preservation or treating agent, especially wood used for exterior applications and/or for contact with moisture.
The amount of deterioration of wood exposed to exterior environmental condition varies widely, depending on the type of wood involved. For example, it is well known that of the woods commonly found in the United States, redwood and cedar are examples of those which have better than average weathering characteristics. For this reason, these woods are often used for such applications as shingles, patio furniture, decking, fence posts, etc. On the other hand, woods such as pine and fir have poor exterior weathering characteristics, and typically, these woods must be treated (and usually constantly retreated) in some manner or used in applications where preservation is not required. It is unfortunate, but readily apparent from the foregoing examples, that the better quality weathering woods are more expensive and less available, while those woods which are cheaper and more abundant are not as desirable from a deterioration standpoint.
It is also known that certain woods and barks have extremely good weathering properties. Such woods and barks include acacia negra, quebracho, mangrove, eucalyptus marginata, chestnut oak, cedar, etc.
The use of substances from such decay and weather resistant woods or barks to impart improved properties to other types of wood is known. For example, in U.S. Pat. No. 3,635 issued June 24, 1844 to Specher for "Improvement in Coloring and Hardening Wood", tannin or tannic acid from oak bark or other woods having high tannin levels is extracted by boiling the starting material in a potash lye solution for a time sufficient to extract the tannin. Wood to be treated is then soaked for several weeks in the resultant solution to impregnate the wood with the tannin-containing extract. Clear water soaking is then used to remove the potash. Iron or copper salts may be used to help darken the wood by immersing the impregnated material into solutions of such salts. The salts are suggested solely as coloring agents. The patent also suggests the same treatment for improving the properties of manila, grass, hemp and other cordage material, and ship rigging. The patent does not teach or suggest the extract and fixation processes of the present invention and, in fact, does not recognize the leaching and fungi attack problems which have now been demonstrated for woods which are merely soaked in tannic acid or tannin.
Another approach to wood preservation is described in U.S. Pat. No. 70,761, issued to Taylor on Nov. 12, 1867 for "Improved Process of Preventing Wood Decay". The patent describes the use of an extraction of oak bark in combination with creosote, acetic acid, wood vinegar, lamp black and kerosene to increase the decay resistance of a wood product. It is suggested that pressure or heat will improve the impregnation and that the tannin content is increased by coagulation thereof with the albumin in the wood. The process is carried out in a retort. In addition to using materials now known to be undesirable from environmental and safety standpoints, the leaching phenomenon and other aspects of the present invention are not considered in the teachings of this patent.
In U.S. Pat. No. 84,733 issued Dec. 8, 1868 to Cowling for "Improvement in Preserving Wood", tannin is mentioned as one substance which can be added to superheated steam to improve the properties of wood. The other suggested substances include coal tar, resins, petroleum, linseed oil, salt, creosote and other analogous compounds.
In U.S. Pat. No. 107,904 issued Oct. 4, 1870 to Hayes for "Improvements in Preserving Wood", tannic acid is added to steam and passed into an airtight chamber to impregnate wood with additional tannin, to form a leathering substance in the wood fiber and to form insoluble compounds with the wood organic matter. The patent relates primarily to the improved impregnation resulting from the use of steam. The patent also indicates that any substance containing tannin may be used, but does not suggest the source of such materials.
In U.S. Pat. No. 142,892 issued Sept. 16, 1873 to Bretonniere for "Improvement In Producing Coloring Matter From Vegetable Substances", tannic acid is mentioned as one of a long list of substances which may be charged into a wood-coloring material in combination with alkaline sulfurates.
In U.S. Pat. No. 216,589 issued June 17, 1879 to Wellhouse, et al for "Improvement In Preserving Wood", a process is described which comprises subjecting wood first to a solution of zinc chloride and gelatins and then to a solution of tannin to render the gelatin insoluble.
William Augustus Hall was issued nine United States patents between 1908 and 1910 relating to wood coloring and preservation. In U.S. Pat. No. 901,095 issued Oct. 13, 1908, for "Art Of Coloring Wood", a process is described which comprises first boiling the wood under pressure in water to soften the resinous products in the wood, and then rendering the wood absorbent by applying vacuum to remove the water. A coloring matter containing tannin salts and extracts of logwood or hemlock are added under hydraulic pressure. The initial boiling step is used to darken the wood, and the tannin and extract are used to further darken and develop the grain of the wood.
In his U.S. Pat. No. 901,096 issued Oct. 13, 1908 for "Art Of Treating Wood For Lumber", Hall discloses a different process in which vacuum is first applied to the wood, followed by a coloring solution impregnation step under pressure, and a final boiling step to diffuse the color into the wood. Extracts of spruce bark, hemlock bark, or mahogany bark are suggested as coloring agents.
In U.S. Pat. No. 901,097 issued Oct. 13, 1908 for "Artificially Grained Or Figured Wood And Process Of Producing The Same", Hall describes another process involving vacuum treatment followed by solution impregnation of a pigmented material. The pigment is deposited mainly in the more porous sections of the cheaper wood to be treated, while the denser sections are stained without pigment deposit. The coloring materials which are suggested include alkaline solutions of suitable bark such as hemlock, oak, or spruce, or any suitable anhydride of tannin.
Another process is described in his U.S. Pat. No. 901,098 issued Oct. 13, 1908 for "Art Of Coloring Wood". In this process, the cheaper wood is first subjected to a vacuum step to render it absorbent following which a staining solution is forced into the wood under high pressure. The wood is then boiled or cooked in the staining solution for a "long time" to diffuse the coloring matter. Once again, the suggested stain is a material selected from the group of bark extracts or tannin anhydrides.
In U.S. Pat. No. 901,099, issued Oct. 13, 1908 for "Art Of Coloring Wood", Hall uses salt solutions or a solution of dye-wood or hemlock bark or anhydrides of tannin as the coloring agent, and a process which comprises a first coloration step in a high-temperature solution of the coloring agent followed by a cold coloration step. The cold step contracts the wood and draws the solution into the surface of the wood until it reaches the desired shade.
In U.S. Pat. No. 913,128, issued Feb. 23, 1909 for "Art of Coloring Wood", Hall describes the use of extracts obtained from the wood or bark of trees such as hemlock, oak or chestnut as coloring agents. The process therein described comprises using the coloring agent in an ammoniacal solution which is impregnated into the cheaper wood under high pressure, either with or without prior vacuum exhaustion of air from the wood.
Hall discloses yet a different wood treating process in U.S. Pat. No. 924,770 issued June 15, 1909 for "Art of Treating Wood for Lumber". In this patent, the wood is treated by impregnating it with phlobaphenes of wood barks, such as mahogany, oak, and hemlock. The absorption is assisted by hydraulic pressure of about 250 psi. Wood coloring rather than preservation is the principal subject of this patent. In U.S. Pat. No. 933,435 issued Sept. 7, 1909 for "Art of Coloring Wood", Hall describes a process in which a solution of coloring extract such as that made from the the bark of hemlock, spruce or oak is selectively introduced in varying degrees into the outer and inner surfaces of the wood by using penetrating substances having different penetrating capabilities and by varying the capillarity of the wood.
In his U.S. Pat. No. 964,017 issued July 12, 1910 for "Art of Transforming Wood", Hall teaches yet another method for changing the color of wood. This method comprises using "colorless" salts in solution to treat wood. The salts suggested included ferrous salts, sodium carbonate and other similar salts. It is suggested that the salts react with the tannic acid contained in the wood to darken the wood, and that the wood will become darkest in the areas of highest tannic acid concentration, e.g., in the areas of the annular rings.
De Cew, in his U.S. Pat. No. 1,010,122 issued Nov. 28, 1911 for Preserved Wood and Process for Making Same", suggests the use of waste sulfite liquor to harden wood and to increase its flame-resistant properties. De Cew also suggests that such treatment results in an insolubilization of the albumin in the wood by the tanning properties of the waste sulfite liquor. A similar process is described in the Mar. 25, 1913 U.S. Pat. No. 1,057,211 issued to Baekeland for "Method of Impregnating Wood and Products Thereof".
In U.S. Pat. No. 1,670,086 issued May 15, 1928, Walker discloses a process for making marine pilings more resistant to marine borers. The process involves impregnating the pilings with a solution of an organic dye such as crystal violet, chrysoidine, or malachite green. It is suggested by the patentee that these materials combine with certain substances in the wood, e.g., cellulose, lignin or tannin to form a new substance which is lethal to the marine borers.
Mengel, in his U.S. Pat. No. 1,774,940 issued Sept. 2, 1930 for "Coloring Walnut Veneer Sheets", describes a process for coloring the sap wood of black walnut veneer sheeting to conform the color to that of the heartwood. The coloring is accomplished with a solution of hot water, chestnut extract (or tannic acid in any form), and a small amount of an iron salt such as ferrous sulfate or ferrous chloride. The veneer is dried and pressed after coloring.
A "Wood Treatment and Product" are disclosed in U.S. Pat. No. 2,062,897 issued to Goodale on December 1, 1936. Arsenic compounds and a protective colloid compatible with the tannin in the wood to be treated are impregnated into the wood to make it more resistant to attack by marine borers, termites, and the like.
Forty-three years later, on Feb. 21, 1978, U.S. Pat. No. 4,075,394 was issued to Meyer for "Process of Inhibiting Tannin Migration in Tannin-Containing Wood Substrates". The process comprises treating a substrate with an aqueous solution of a polyalkylenimine, such as polyethylenimine. A related process is described in Meyer, et al. U.S. Pat. No. 4,218,516 issued Aug. 19, 1980 for "Pigment for Blocking Tannin Migration". In the latter, the patentees use a stain-inhibiting amount of magnesium hydroxide-tannin complex in the treated wood. Both patents relate to preventing stain streaking when films, e.g., paints are applied to high-tannin content woods such as redwood, cedar, or mahogany.
Mitchell, et al., in U.S. Pat. No. 4,270,688 issued Sept. 2, 1980 for "Protecting Wood from Wood Degrading Organisms", describe a method for treating wood by first pressure-impregnating the wood in a tannic acid-ethanol solution, followed by pressure-impregnation with a solution of a metal salt which will complex with both the impregnated tannic acid and with the wood. The process is directed to preventing attack of wood by degrading organisms such as termites, fungi and marine borers. The tannic acid impregnation step uses concentration of from 5-50%, while the metal salt is used in concentrations of 1-50%. The tannic acid or salt solutions can be impregnated under pressures of from 20-100 psi.
In addition to the above-mentioned patents which refer to wood treatment, the present inventors are aware that certain tannin containing extracts are being, and have been, used for a number of years for tanning leather, a process which in and of itself is well known. Many of such extracts which are now being used in the United States are derived from various parts of plant species which are foreign to the United States. Some of such extracts are derived from some of the same woods which supply the weathering resistant woods currently being imported, reference to which has previously been made in this section of the specification. Such extracts perform the preservative function of tanning leather and in addition, provide a coloring function. Such extracts are currently being used to treat fishing nets, but are also used for such diverse purposes as additives for drilling well muds, etc.
While research has been underway for more than a century to discover ways of converting relatively cheaper woods into wood products which are resistant to weathering, fungi, borers, insects, etc., the present inventors are not aware of any commercially viable treatment systems, except those involving environmentally damaging substances such as creosote, arsenic such as in certain proprietary mixtures of ammonia, chromium and arsenic compounds referred to as ACA or certain proprietary mixtures of copper, chromium and arsenic compounds referred to as CCA, (such as ammonium-chromium-arsenate (ACA) or copper-chromium-arsenate (CCA)), or pentachlorophenol. A method for treating wood in a commercially and environmentally suitable process, and the wood product prepared and sometimes aesthetically enhanced by such method would represent substantial advances in the art.